An ablation catheter used to treat arrhythmia by cauterizing a portion of cardiac muscle causing arrhythmia with a high-frequency electric current is known. On the other hand, an operation method for externally treating the heart has been proposed, which involves percutaneously inserting a treatment device into a pericardial cavity, which is a space between the heart and the pericardium, from near the xiphisternum. When an ablation catheter is used to cauterize the cardiac muscle from the outside of the heart, the ablation catheter cannot be brought close enough to the cardiac muscle because of fat covering the heart surface, so it is difficult to efficiently supply a high-frequency electric current to the cardiac muscle. To counter this problem, an apparatus for melting and removing fat with ultrasonic waves is used (for example, see PTL 1).